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Objectives: A modern pattern (rate and duration) of dental development occurs relatively recently during human evolution. Given the temporal overlap of Homo naledi with the first appearance of fossil Homo sapiens in Africa, this small-bodied and small-brained hominin presents an opportunity to elucidate the evolution of enamel growth in the hominin clade. Here we conduct the first histological study of two permanent mandibular canines and one permanent maxillary first molar, representing three individuals attributed to H. naledi. We reconstruct the rate and duration of enamel growth and compare these findings to those reported for other fossil hominins and recent humans. Materials and Methods: Thin sections of each tooth were produced using standard histological methods. Daily and longer period incremental markings were measured to reconstruct enamel secretion and extension rates, Retzius periodicity, canine crown and molar cusp formation time. Results: Daily enamel secretion rates overlapped with those from recent hominins. Canine crown formation time is similar to that observed in recent Europeans but is longer than canine formation times reported for most other hominins including Australopithecus and H. neanderthalensis. The extended period of canine formation appears to be due to a relatively tall enamel crown and a sustained slow rate of enamel extension in the cervical portion of the crown. A Retzius periodicity of 11 days for the canines, and nine days for the molar, in H. naledi parallel results found in recent humans. An 11-day periodicity has not been reported for Late Pleistocene Homo (H. erectus, H. neanderthalensis) and is rarely found in Australopithecus and Paranthropus species. Discussion: Enamel growth of H. naledi is most similar to recent humans though comparative data are limited for most fossil hominin species. The high Retzius periodicity values do not follow expectations for a small-brained hominin.

We thank the many funding agencies that supported this work, in particular the National Geographic Society, the National Research Foundation, the Lyda Hill Foundation, and the Lee R. Berger Foundation for Exploration for particularly significant funding of the discovery, recovery, and analysis of this material. DGM and MB were funded through CGL2012-37279, MINECO, Spain, and a Leakey Research Grant to DGM. A visit to the Royal Museum for Central Africa (RMCA; Tervuren, Belgium) by DGM was funded by the European Commission's Research Infrastructure Action via the Synthesys Project (BE-TAF-5639). SAW was funded through the New York University Research Challenge Fund., Hominin evolution featured shifts from a trunk shape suitable for climbing and housing a large gut to a trunk adapted to bipedalism and higher quality diets. Our knowledge regarding the tempo, mode, and context in which these derived traits evolved has been limited, based largely on a small-bodied Australopithecus partial skeleton (A.L. 288-1; “Lucy”) and a juvenile Homo erectus skeleton (KNM-WT 15000; “Turkana Boy”). Two recent discoveries, of a large-bodied Australopithecus afarensis (KSD-VP-1/1) and two Australopithecus sediba partial skeletons (MH1 and MH2), have added to our understanding of thorax evolution; however, little is known about thorax morphology in early Homo. Here we describe hominin vertebrae, ribs, and sternal remains from the Dinaledi chamber of the Rising Star cave system attributed to Homo naledi. Although the remains are highly fragmented, the best-preserved specimens—two lower thoracic vertebrae and a lower rib—were found in association and belong to a small-bodied individual. A second lower rib may belong to this individual as well. All four of these individual elements are amongst the smallest known in the hominin fossil record. H. naledi is characterized by robust, relatively uncurved lower ribs and a relatively large spinal canal. We expect that the recovery of additional material from Rising Star Cave will clarify the nature of these traits and shed light on H. naledi functional morphology and phylogeny., Depto. de Biodiversidad, Ecología y Evolución, Fac. de Ciencias Biológicas, TRUE, pub

Homo naledi es una especie previamente desconocida de homínido extinto descubierta dentro de la Cámara Dinaledi del sistema de cuevas Rising Star, en el Cradle of Humankind, Sudáfrica. Esta especie se caracteriza por tener una masa corporal y estatura similar a las poblaciones humanas de cuerpo pequeño, pero un volumen endocraneal pequeño similar al de los australopitecos. La morfología craneal de H. naledi es única, pero se asemeja más a las especies tempranas de Homo, incluyendo Homo erectus, Homo habilis o Homo rudolfensis. Aunque primitiva, la dentición es generalmente pequeña y simple en su morfología oclusal. H. naledi presenta adaptaciones manipulativas de la mano y la muñeca similares a las humanas. También exhibe un pie y miembro inferior similares a los humanos. Estos aspectos humanos se contrastan en el postcráneo con un tronco, hombro, pelvis y fémur proximal más primitivos o similares a los australopitecos. Representando al menos 15 individuos con la mayoría de los elementos esqueléticos repetidos múltiples veces, esta es la mayor colección de una sola especie de homínidos descubierta en África., National Geographic Society, The National Research Foundation of South Africa (WARF), The Palaeontological Scientific Trust, Lyda Hill Foundation, Wisconsin Alumni Research Foundation (WARF), Texas A and M University, Depto. de Biodiversidad, Ecología y Evolución, Fac. de Ciencias Biológicas, TRUE, pub